The present invention relates to automatic exposure control and automatic gain control which are performed by an image pickup apparatus of an electronic still camera or the like.
More particularly, the present invention relates to an operation of a camera which is performed when a shutter is operated.
FIG. 21 a diagram showing the structure of automatic exposure adjustment for an image pickup apparatus disclosed in Japanese Patent Publication No. Hei 5-37595. Referring to FIG. 21, reference numeral 20 represents a lens, 21 represents an aperture mechanism, 22 represents a drive circuit, 23 represents an image pickup device, 24 represents a preamplifier, represents a variable gain amplifier, 26 represents an A/D converter, 27 represents a synchronization separation circuit, 28 represents a switching circuit, 29 represents a switching control circuit, 30 represents an integrating circuit, 31 represents an abnormality determination circuit, 32 represents an average value calculating circuit, 33 represents a comparison circuit, 34 represents a desired-value memory and 35 represents a reference-value memory.
The operation will now be described. The quantity of light made incident from a subject on the lens 20 is adjusted because the aperture of the lens 20 is adjusted by the aperture mechanism 21. Then, incident light is photoelectrically converted by the image pickup device 23 so as to be transmitted as a picked-up image signal. The A/D converter 26 converts a brightness signal in the picked up image signal into a digital signal which is supplied to a selected integrating circuit 30 by the switching circuit 28.
The synchronization separation circuit 27 separates the picked up image signal into vertical and horizontal synchronizing signals. The following switching control circuit 29 controls the switching operation of the switching circuit 28 in response to clock signals which have been used to control the two synchronizing signals and the image pickup device 23. Thus, switching is performed to correspond to the divided sampling areas, as shown in FIG. 22. That is, a value obtained by A/D-converting the picked up image signal in area A is supplied to an integrating circuit 30A. Similarly, values obtained by A/D-converting the image signals in areas B, D, D, E and F are supplied to integrating circuits 30B, 30C, 30D, 30E and 30F.
Each of the integrating circuits 30A to 30F integrates supplied image signals for one field so as to transmit a result of integration to the abnormality determination circuit 31. Abnormality determination circuits 31A to 31F delete integrated values in areas each including brightness not higher than a constant level determined by the reference-value memory 35 and brightness not lower than a predetermined level to transmit results to the average value calculating circuit 32. The average value calculating circuit 32 calculates average brightness.
The comparison circuit 33 performs comparison with a predetermined brightness level previously stored in the desired-value memory 34. Then, the aperture mechanism 21 and the variable gain amplifier 25 are controlled in such a manner that the average brightness approaches the determined brightness level stored in the desired-value memory 34.
As described above, the conventional image pickup apparatuses have mainly employed TTL metering which requires the comparison circuit for causing the integrated value of an image signal to approach a predetermined level and arranged to adjust exposure in accordance with a result of the comparison performed by the comparison circuit. Although the above-mentioned conventional example comprises the mechanical aperture for adjusting exposure, a similar structure is constituted even if an image pickup device having a variable charge storage time is employed.
In addition to the TTL metering method, an external metering method is known which uses a photometry sensor for measuring the brightness of a picked up image similarly to the metering operation which is performed by the silver salt film, as shown in FIG. 23. Referring to FIG. 23, the same reference numerals as those shown in FIG. 21 represent the same or corresponding elements. Reference numeral 42 represents an A/D converter and 43 represents a microcomputer.
The operation will now be described.
The quantity of incident light converged by the lens 20 is adjusted by the aperture mechanism 21 so that the picked up image is formed on the image pickup device 23. The variable gain amplifier 25 amplifiers a signal transmitted from the image pickup device 23, and then transmits an image signal. A lens 40 for a photometry sensor is disposed in such a manner that an image is formed on the photometry sensor 40 with the same field angle as that of the image formed on the image pickup apparatus 23. The photometry sensor 41 is arranged to transmit a voltage level which is changed in accordance with the quantity of incident light. The transmitted voltage level is converted into a digital signal by the A/D converter 42, and then supplied to the microcomputer 43. The microcomputer 43 is able to detect the quantity of incident light, that is, the brightness of the subject which is being picked up in accordance with the voltage level supplied from the photometry sensor 41. The microcomputer 43 transmits the predetermined degree of aperture of the aperture mechanism 21 and the gain of the variable gain amplifier 25 in such a manner that a predetermined image signal can be obtained in accordance with the value obtained from the photometry sensor 41. Moreover, the microcomputer 43 controls the exposure in such a manner that the level of the image signal which is transmitted from the variable gain amplifier 25 is made to be constant.
The two examples of the exposure control method adapted to the conventional image pickup apparatuses have been described above. Video cameras and electronic still cameras usually employ the TTL metering method shown in FIG. 21 or the external metering method shown in FIG. 23 to control the exposure.
The exposure control which is performed by the conventional image pickup apparatus employing the TTL metering method is arranged in such a manner that the integrated value in each field is subjected to a comparison in the comparison circuit at each operation so as to converge the obtained values in such a manner that a constant brightness level can be realized. To realize a predetermined brightness level, comparison must be performed to correspond to the steps of the degrees of the aperture. Therefore, a long time is required to realize a predetermined image signal level. If the steps of the degree of the aperture is enlarged to shorten the required time, there arises a problem in that the aperture encounters hunting.
The exposure control which is performed by the external metering method is arranged in such a manner that an image similar to that obtainable by the image pickup device is formed on the photometry sensor. Therefore, a precise photometry sensor lens is required. Since the image pickup device has a narrow input dynamic range as compared with that of the silver salt camera, precise exposure control must be performed. Therefore, the accuracy of the aperture mechanism and environmental dependency, such as the temperature characteristic of the photometry sensor, must be considered. As a result, there arises a problem in that the structure becomes too complicated.
What is worse, also the exposure control which is performed by the TTL metering method is required to continuously operate all of the circuits until a required image is picked up. Therefore, there arises a problem in that a battery is consumed extremely quickly if electric power is supplied from the battery.
An object of the present invention, attempted to solve the above-mentioned problems, is to obtain an image pickup apparatus which is capable of performing precise exposure control with a simple circuit structure and which does not required large power consumption.
An image pickup apparatus according to the present invention has a first LUT composed of table values of brightness of N subjects determined to make the integrated value to be constant and a second LUT composed of table values of N charge storage time periods corresponding to the table values of the brightness of the N subjects. When an image has been picked up with the charge storage time at the n-th address in the second LUT, exposure control is performed in such a manner that the integrated value obtained from the integrating means is divided by the n-th charge storage time so that the brightness of the subject is calculated. Then, charge storage time corresponding to the calculated brightness of the subject is selected from the second LUT.
Moreover, the image pickup apparatus has a third LUT composed of table values of brightness of N subjects determined to make the integrated value to be constant, a fourth LUT composed of table values of N charge storage time periods corresponding to the table values of the brightness of the N subjects and a fifth LUT composed of table values which are gains of N gain adjustment means. When an image has been picked up with the n-th charge storage time and gain in the third and fourth LUT, the integrated value obtained from the integrating means is divided by the n-th charge storage time and gain so that the brightness of the subject is calculated. Moreover, the charge storage time and gain corresponding to the calculated brightness of the subject are selected from the third and fourth LUT so that the exposure control is performed.
Moreover, sixth and seventh LUT having table values which are logarithmic values of table values of the second LUT which are exponentially increased/decreased and which are linearly increased/decreased are provided. The table values which are linearly charged are used to calculate the brightness of the subject.
Moreover, seventh, eighth and ninth LUT having table values which are logarithmic values of table values of the fourth and fifth LUT which are exponentially increased/decreased and which are linearly increased/decreased are provided. The table values which are linearly increased/decreased are used to calculate the brightness of the subject.
The image pickup apparatus has a first power supply circuit for supplying electric power to the solid state image device, the timing generator, the gain adjustment means and the integrating means, a second power supply circuit for supplying electric power to the calculating means and the recording means, and a shutter button having first and second switch mechanisms. When the first switch mechanism has been switched on, electric power is supplied to the image pickup circuit from the first power supply circuit so perform exposure adjustment and gain adjustment operation m times. When the second switch mechanism has been switched on during the m times of the exposure adjustment and the gain adjustment operations, the foregoing operations are performed m times, and an image is picked up by using the charge storage time and gain at the m-th operation. If the second switch mechanism is not switched on during the m times of the exposure adjustment and the gain adjustment operations, the charge storage time and gain at the m-th time are recorded and stored, and supply of electric power from the first power source to the image pickup circuit is interrupted. When the second switch mechanism has been switched on, electric power is again supplied from the first power source to the image pickup circuit. Thus, the image is picked up with the recorded and stored charge storage time and gain at the m-th time.